|Message: Re: How is parallelization implemented using MPI and Geant4?||Not Logged In (login)|
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Most programs based on Geant4 are trivially parallelizable, in that each event is independent of the others, so one merely needs to distribute the events to the different parallel jobs. One needs to arrange for the output files of the parallel jobs to be different, and one needs a way to combine them afterward. Note that Geant4 is not currently capable of parallelizing tracking within an event, though I believe there is a group looking at it; I have looked at it (in another context) and there are many difficulties.
In my program, G4beamline, I seed the random number generator with the event number before each event, so the user need merely arrange for the different jobs to use disjoint event numbers, and to name each job's output file using the first event number. By outputting NTuples into Root files, combining the results is simple. Note that MPI is not needed at all -- a simple shell script starts all the jobs and Root combines the output files into plots and histograms.
This works for parallelization up to perhaps 32 jobs. Beyond that it gets rather cumbersome; certainly 10,000 parallel jobs is not feasible this way.
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